unix: add support for receiving UNIX sockets
unix: add support for receiving UNIX sockets

Until now, all received file descriptors were reported as being UV_TCP streams:
AF_INET/AF_INET6 sockets of type SOCK_STREAM.

This commit adds support for AF_UNIX/AF_FILE sockets (UV_NAMED_PIPE in libuv
nomenclature).

Support for UV_UDP handles (AF_INET/AF_INET6 sockets of type SOCK_DGRAM) still
needs to be implemented.

urbit/archaeology2,urbit/archaeology2,urbit/archaeology2,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,urbit/archaeology2,unknownbrackets/maxcso,urbit/archaeology2,unknownbrackets/maxcso,urbit/archaeology2,unknownbrackets/maxcso,urbit/archaeology2,urbit/archaeology2,urbit/archaeology2